Starting at the end of the nineteenth century, crime fighters began to use and develop what has grown into a substantial body of technological tools designed to detect and/or enhance physical evidence. One of the earliest techniques of this kind to receive widespread application is the dusting of fingerprints. Light sources were also among the first tools used in this field. Hence the classic icon of the gumshoe, flashlight in hand, support for evidence at the dimly lit crime scene.
When a fingerprint is fresh, the oil which forms the print generally follows the pattern of the fingerprint ridges in the finger which made the print. If a fine dust is applied to the surface of a fresh print, the dust tends to adhere to the oils in the fingerprint, thus forming a pattern which generally reveals the pattern of the fingerprint.
Fingerprint dusts were initially selected for their color contrasting qualities as compared to the background. Thus white dust was used to enhance a fingerprint on a black object and vise versa. However where the oils of a fingerprint have lost their tackiness due to aging or other phenomena, the amino acids into which they break down do cause a minute etching of many surfaces. While this etching is often not visible to the naked eye, and may not become visible with the application of a colored powder, extremely fine florescent dusting powders will reveal the fingerprint pattern when illuminated under high intensity light.
Today when florescent dusting powders are used, inspection of the evidence is done with specialized light sources. These light source usually comprise a high intensity source and a filter which passes light having a limited range of wavelengths. Depending upon the material used, which material may be either a florescent dusting powder, dye, or other marker material, light having a wavelength which substantially coincides with a known excitation frequency of the marker is employed. The characteristic of the marker is that, upon illumination with light at one of its excitation wavelengths, it will fluoresce, or emit light. Such fluorescence is typically at a wavelength different from the excitation wavelength.
Examination of evidence is also enhanced through the use of color filtering glasses or barrier filters, whose color filtering characteristics are tuned to maximize the image to be detected. As noted above, the excitation wavelength is varied through the use of filters at the source. While such devices are very efficient in filtering light, every filter has its own fixed characteristics. These include its center wavelength, bandwidth and transmission coefficient. Thus, if one wishes to have flexibility, it is necessary to have a wide range of filters having different center wavelengths and different bandwidths. This is both cumbersome and expensive. Moreover, as new dyes and powders are introduced, old filters can become obsolete or unnecessary.
In an attempt to address this problem, some light sources used for forensic examination come with a mechanical filter assembly, which allows the introduction of one of about a half dozen filters into the path of the light source to provide the desired wavelength illumination. While this does solve the problem of providing a convenient and easy way to use a light source, obsolescence and limited wavelength and bandwidth selection remain.
In an attempt to overcome some of these disadvantages earlier forensic illumination systems have attempted to achieve a measure of tunability by mounting an interference filter for angular rotation. Generally, such angular rotation results in a change in angle of incidence with respect to the filter input and a relatively small variation in the encountered path length between the functional layers in the interference filterfor light passing through the filter in a fixed direction. In accordance with Bragg's Law, this results in different wavelength filtering characteristics.